Schematic illustration of four steps during leaf development. (A) The founder cells of leaf primordium are recruited from the flank of the SAM. I1, the oldest incipient primordium. (B) After initiation, the leaf primordium grows predominantly in the distal direction and establishes three growth axes: adaxial-abaxial axis (Ad-Ab), proximal-distal axis (Pr-Di), and medio-lateral axis (Me-La). Left, front view. Right, top view. (C) Afterwards, the blade initiates from the marginal regions and grows along medial-lateral axis to separate blade from petiole. (D) Finally, the blade undergoes rapid expansion, which is accompanied by cell division and cell differentiation occurring in the entire blade. The transition from cell division to differentiation shift basipetally in Arabidopsis but can be divergent in other species. Red arrows in (A–C) indicate the direction of leaf growth. Red lines in (D) indicate the arrest front. Modified from [,].

Models of the regulation of leaf initiation and early patterning by auxin. (A) Model of the regulation of leaf patterning by auxin polar transport in the epidermis and spatial auxin distribution. Note that auxin maxima are also auxin sinks, where PIN1 directs auxin transport into inner cells. Modified from [,]; (B) Model of the regulation of leaf patterning by spatial auxin distribution and signaling. Modified from [].

Morphology of simple and compound leaves. (A) Leaves of Arabidopsis, Alfalfa (Medicago sativa) and tomato. SE, serration; PE, petiole; TL, terminal leaflet; LL, lateral leaflet; R, rachis; S, stipule; L, lobe. Scale bars: 1 cm. (B) Model of the regulation of compound leaf development by auxin, GOB, E, and E-targeted ARFs in tomato.